1. Field of the Invention
The present invention generally relates to a video signal processing circuit applicable in a luminance-chrominance signal separator for separating a composite color video signal, such as used in a magnetic tape recording and/or reproducing apparatus, into a luminance signal and a chrominance signal and, more particularly, to the video signal processing circuit effective to provide a chrominance signal substantially free from a color displacement in a vertical direction.
2. Description of the Prior Art
In discussing the prior art to the present invention, reference will now be made to a luminance-chrominance signal separator used in a reproducing system of the magnetic tape recording and/or reproducing apparatus, which apparatus is hereinafter referred to as a video tape recorder.
FIG. 8 of the accompanying drawings illustrates a block circuit diagram of the prior art luminance-chrominance signal separating circuit disclosed in a Japanese book entitled "Digital Signal Processing of Images", pages 109 to 110, authored by Takahiko Fukinuke and .published from Nikkan Kogyo Newspapers Publishing Co., Ltd. Referring to FIG. 8, the luminance-chrominance signal separating circuit shown therein comprises a 1H delay circuit having a delay time equal to one horizontal scanning period, bandpass filters 3 and 4 for passing a chrominance signal of predetermined bandwidth, a first subtractor 24, an amplifier 9 having a 1/2 gain and a second subtractor 25.
The luminance-chrominance signal separating circuitt of the construction shown in FIG. 8 operates in the following manner. Let it be assumed that a composite color video signal inputted is of a type containing, as shown in FIGS. 10(a) to 10(d), red color signal components until (n-2)H line and containing no color signal subsequent to the (n-1)H line, that is, of a type representing, as shown in FIG. 11(a), a color picture having a division of red and white portions. Since the carrier wave frequency of the chrominance signal is chosen to be a frequency that is inverted for each line, the chrominance signals have their phase displaced 180.degree. from each other as shown in FIGS. 10(a) and 10(b). Assuming that the composite color video signal inputted at the timing of (n-2)H as shown in FIG. 10(a) is being processed to separate it into the luminance signal and the chrominance signal, the composite color video signal inputted to the 1H delay circuit 1 shown in FIG. 8 at the timing of (n-2 )H is delayed a predetermined time equal to one horizontal scanning period (1H) to provide a composite color video signal for (n-3)H line (i.e., having the same phase as shown in FIG. 10(a)) which is subsequently inputted to the bandpass filter 4. The bandpass filter 4 then outputs a chrominance signal component of (n-3)H line. On the other hand, the composite color video signal of (n-2)H is inputted to the bandpass filter 3 from which a chrominance signal component of (n-2)H is outputted. The subtractor 24 performs a subtraction of [(n-2)H-(n-3)H] and subsequently outputs a difference signal descriptive of a result of the subtraction which is then amplified by the amplifier 9 by a gain of 1/2. The amplifier 9 then outputs a chrominance signal component of (n-2)H as shown in FIG. 10(j).
On the other hand, the subtractor 25 subtracts the chrominance signal component of (n-2)H from the composite color video signal of (n-2)H, thereby outputting a luminance signal component of (n-2)H as shown in FIG. 10(f). Thus, the composite color video signal of (n-2)H as shown in FIG. 10(b) is separated into the luminance signal component of (n-2)H as shown in FIG. 10(f) and the chrominance signal component of (n-2)H as shown in FIG. 10(j). Similarly, the composite color video signal containing the red chrominance signal as shown in FIG. 10(a) is separated into the luminance signal component of (n-3)H as shown in FIG. 10(e) and the chrominance signal component of (n-3)H as shown in FIG. 10(i).
The composite color video signal of (n-1)H as shown in FIG. 10(c) is Similarly arithmetically processed with the composite color video signal of (n-2)H which has been delayed a 1H period so that it can be separated into the luminance signal component of (n-1)H as shown in FIG. 10(g) and the chrominance signal component of (n-1)H as shown in FIG. 10(k), and the composite color video signal of nH as shown in FIG. 10(d) is similarly arithmetically processed with the composite color video signal of (n-1)H as shown in FIG. 10(c) and the composite color video signal of (n-1)H, which has been delayed a 1H period so that it can be separated into the luminance signal component of nH as shown in FIG. 10(h) and the chrominance signal component of nH as shown in FIG. 10(l).
The frequency characteristics of the prior art separating circuit of the type referred to above will now be described.
The transfer function Hc of the chrominance signal path within the bandwidth of each of the bandpass filters 3 and 4 and the transfer function H, of the luminance signal path within the bandwidth of each of the bandpass filters 3 and 4 can be expressed by the following equations. EQU Hc=(1-Z.sup.-1)/2 EQU H.sub.Y =(1+Z.sup.-1)/2
Since Z.sup.-1 =.sup.-jwt wherein T represents one horizontal scanning period, the transfer function H.sub.Y can be rewritten as follows. EQU H.sub.Y =[(1+cos.omega.T)/2]-(sin.omega.T).multidot.j/2
The gain frequency characteristic .vertline.H.sub.Y .vertline. can be expressed by the following equation. ##EQU1##
The gain frequency characteristic .vertline.H.sub.Y .vertline. attains a maximum value when .omega.=2n.pi./T and a minimum value when .omega.=(2n+1).pi./T, wherein n represents an integer. Therefore, the prior art separating circuit shown in FIG. 8 has such a combshaped characteristic that it attains the maximum and minimum values at a cycle of 1/T within the bandwidth of each of the bandpass filters 3 and 4 and that no chrominance signal component will not be outputted outside the bandwidth of each of the bandpass filters 3 and 4 and, therefore, the transfer function H.sub.Y of the luminance signal component will be 1, and the overall frequency characteristic thereof will be such as shown in FIG. 9.
However, where the separating circuit is so constructed as shown in FIG. 8, and in the event that the color of the composite signal in the vertical direction varies steeply as shown in FIG. 11(a), such a phenomenon in which the chrominance signal displaces by 1H line as shown in FIG. 11(b) by the following reason. This can readily be understood when the separation of the composite signal of (n-1)H into the luminance signal and the chrominance signal is taken into consideration.
Since an output from the subtractor 24 generated at a timing of (n-1) corresponds to the chrominance signal component (as shown in FIG. 10(j)) of (n-2)H from which the chrominance signal component (zero) of (n-1)H has been subtracted, an output from the 1/2 amplifier 9 is in a phase opposite to that of the chrominance signal component of (n-2)H, as shown in FIG. 10(k) and represents the chrominance signal component whose level is 1/2 of the chrominance signal component of (n-2)H, and is outputted as the chrominance signal component of (n-1)H. On the other hand, the luminance signal component of (n-1)H is outputted as a luminance signal component of (n-1)H as shown in FIG. 10(g).
Thus, the chrominance signal having an amplitude corresponding to 1/2 of the chrominance signal component of composite signal of (n-2)H line preceding 1H period is outputted to a (n-1)H line in which no chrominance signal exists originally and, as shown in FIG. 11(b), at the point at which a red portion changes to a white portion, a line colored red in one line is reproduced.
In a video tape recording and/or reproducing apparatus, or a video tape player, for home use, the signal is passed a number of times through the luminance-chrominance separating circuit, a line correlation noise cancelling circuit having a structure similar thereto and a chrominance signal cross-talk cancelling circuit and, therefore, a problem has been found existing in that a color displacement corresponding to several lines tends to occur.
In recent years, the horizontal resolution of the video tape player has been satisfactorily improved to provide a high quality image reproduction, however, the vertical resolution thereof tends to pose a problem to the accomplishment of the high quality image reproduction because of the previously discussed phenomenon.
On the other hand, the U.S. Pat. No. 4,789,890, issued Dec. 6, 1988, and the U.S. Pat. No. 4,141,035, issued Feb. 20, 1979, to Maxemchuk et al. disclose a composite video signal separating system wherein the vertical filter and the horizontal filter are selectively brought into operation to accomplish the separation of the composite video signal into the luminance signal and the chrominance signal. The Japanese laid-open Patent Publication No. 58-129890, published Aug. 3, 1983, discloses a system wherein the relationship between horizontal, vertical and oblique correlations are detected by the use of a chrominance signal separating filter. The U.S. Pat. No. 4,489,346, issued Dec. 18, 1984 to Tanaka et al. discloses the separation of the composite video signal into the chrominance and luminance signal components by converting data classified in a pattern space.
Although the technique disclosed in any one of the above discussed prior art references appears effective to minimize the color displacement to some extent, it requires a complicated and expensive construction to accomplish the separation of the composite video signal into the chrominance and luminance signal components.